52 



PROTOZOIC ROCKS 



portion of the sections at Prairie du Chien ; so that there was an entire absence of 

 all palgeontological evidence as to the exact place which these strata occupied in 

 the "Western geological series. It is, therefore, with no small degree of satisfaction 

 that I find myself able to disclose a new feature in the palaeontology of Western 

 America, and thus to furnish, not only to the geologist a key to the stratigraphical 

 position of the rocks north of the Wisconsin River, but, at the same time, to the 

 miner his surest and safest guide by which to direct operations in the search after 

 mineral wealth. 



The following table exhibits the most persistent elements of stratification of this 

 Great Sandstone Formation ; showing the relative position of the different Trilobite 

 beds, and the thickness of the intervening strata. 



TABLE OF THE ELEMENTARY STRATIFICATION OF THE LOWEST PROTOZOIC 



SANDSTONES. FORMATION 1. 



Sixth Trilobite 

 bed. 



Quartzose, light-coloured sandstones, of various degrees of indu- 

 ration, with intercalations of beds of Magnesian Limestone, 

 with glistening crystalline facets, and calcareo-siliceous 

 oolite, produced by rounded grains of quartz, encased in cal- 

 careous cement, containing Euompludus and imperfect Tri- 

 lobites. Locally, with a band of green earth, . 



Mammillary and botryoidal layer of white sandstone ; some- 

 times banded with yellow, ...... 



Thick beds of soft, yellowish, and brown sandstone, sometimes 

 with botryoidal, hard, projecting concretions, passing down- 

 wards into fine-grained, soft sandstones, approaching tripoli, 



Feet. 



Inehes. 



50 to 85 



5 to 6 



40 to 50 



Fifth Trilobite C Ash-coloured an d yellowish argillo-calcareous and magnesio- 

 • j 1 calcareous beds, containing Dihelocephalus 31innesotensis. 



v. Stillwater Trilobite bed, ....... 



Green, red, and yellowish sandstones, with thin, schistose, 

 dolomitic intercalations, ....... 



Upper, brown dolomitic layers, containing Orihis, Lingulas, 

 and columns of Crinoidea, as at La Grange Mountain, 

 Fourth Trilobite ( Alternations of yellow, laminated sandstones, with green 



bed. 



d -> 



Third Trilobite 

 bed. 



< particles disseminated, 



(. Marine Mill Trilobite grit, 



Fucoidal layers, and thin-bedded green and yellow sandstones ; 

 at their base; often a band of about six inches of green earth, 

 used by the Indians as a pigment, ..... 



Green and red sandstones, charged with silicate of iron, 

 Loose, green sand, and soft, green sandstone, 

 Micaceous sandstone, containing Dihelocephalus Menishaensis, 

 D. granulosa, &c, ........ 



Alternations of green and ferruginous sandstones, 

 Micaceous sandstones, containing D. Menishaensis, &c, 

 Thin layers of green sand, alternating with green earth, im- 

 pregnated with silicate of iron, ..... 



Lower, brown siliceo-calcareous and dolomitic bands of Moun- 

 tain Island, and elsewhere, ...... 



Soft, thin-bedded sandstones, with scales of mica disseminated, 



to 10 



40 



4 



5 

 5 



30 



to 40 

 5 



15 



3 

 20 

 o 



30 to 40 



10 



4 



to 15 



